1. Field of the Invention
The present invention relates to a liquid crystal display device in which spacers and a liquid crystal layer are held between a pair of substrates that are disposed to face each other.
2. Description of the Related Art
In general, in a liquid crystal display device, a liquid crystal layer is held between a pair of substrates that are disposed to face each other. In addition, to keep a thickness of the liquid crystal layer (hereinafter, referred to as “a liquid crystal gap”) to be uniform, a plurality of spacers are disposed between the substrates.
For example, in Japanese Patent Application Laid-open No. 2004-61904, a technology is disclosed in which photosensitive resin is used for a material of the spacers, and the spacers are disposed in spaces between respective pixels with an uniform density over an entire substrates by a photolithographic process. Thus, it becomes possible to prevent unevenness of a liquid crystal gap, defective of pixels, and reduction of an aperture ratio.
FIG. 1 is a plan view showing a diagrammatic constitution of a display region in a conventional liquid crystal display device. As shown in this figure, in a display region 1, one pixel is constituted by three single-color sub-pixels, that is, a red sub-pixel, a green sub-pixel, and a blue sub-pixel. In addition, columnar spacers 2a are disposed in spaces between respective pixels, with a uniform density. Reference numeral 10 in FIG. 1 denotes a region where the spacers 2a do not exist. Moreover, symbols R, G, and B in FIG. 1 denote a red sub-pixel, a green sub-pixel, and a blue sub-pixel, respectively.
FIG. 2 is a sectional view of the display region 1 taken along a line A-A in FIG. 1. As shown in this figure, on an array substrate 4, a plurality of pixel electrodes 3 are disposed, and on a counter substrate 7, a plurality of color filters 5 and a common counter electrode 6 are disposed. Between the array substrate 4 and the counter substrate 7, a liquid crystal layer 8 is held with a uniform liquid crystal gap 9, by using the spacers 2a. Although the liquid crystal display device includes, besides those described above, thin-film transistors, alignment films, polarizing plates, and the like, they are omitted in the drawing.
Incidentally, in the liquid crystal display device, the array substrate 4 and the counter substrate 7 are frequently subjected to external forces 11 denoted by arrows shown in FIG. 2 from outside.
In this case, when a density of the spacers 2a in the display region 1 of FIG. 1 is low and a spacer absent region 10 is large, the array substrate 4 and the counter substrate 7 deform beyond an elastic deformation range, and consequently, irreversible deformation occurs in the liquid crystal gap 9. Thus, an uneven display occurs on a display screen, and it becomes a significant defect in display quality for products.
On the other hand, to address such problems, a method has been proposed in which a density of spacers disposed in a display region is increased to enhance a pressure resistance to external forces which are applied to substrates, when a liquid crystal display device is designed.
However, since a thermal expansion coefficient of liquid crystals is one digit higher than those of other components, a thermal shrinkage of the liquid crystals, and those of substrates and spacer components are different to a large extent. Therefore, when a liquid crystal display device is stored or placed under a low temperature environment such as a cold region or an air cargo compartment, where, in some cases, a temperature even goes down to several tens of degrees below zero Celsius, forces work to shrink the liquid crystal gap. Especially, in a case of twisted nematic (TN) liquid crystals, forces work to shrink even several percent of the liquid crystal gap.
In the above case, when spacers are disposed with a high density, the liquid crystal gap cannot be sufficiently shrunk, and consequently, bubbles may possibly occur in the vicinities of spacers. The bubbles are supposed to contain a gas with extremely low-pressure. Hereinafter, the bubbles that could occur under environments where the temperature is lower than a normal temperature are referred to as low temperature bubbles.
There is a problem that the low temperature bubbles, which once have occurred, do not disappear in some cases even after a surrounding environment of a liquid crystal display device is put back to a normal temperature environment, and this problem becomes a cause of defect in display quality.